linux-kernel - Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp

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Date:   Wed, 21 Jun 2023 11:02:05 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Yair Podemsky <ypodemsk@...hat.com>
Cc:     mtosatti@...hat.com, ppandit@...hat.com,
        David Hildenbrand <david@...hat.com>,
        "Russell King (Oracle)" <linux@...linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Heiko Carstens <hca@...ux.ibm.com>, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        svens@...ux.ibm.com, "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, frederic@...nel.org,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, ardb@...nel.org,
        samitolvanen@...gle.com, juerg.haefliger@...onical.com,
        Arnd Bergmann <arnd@...db.de>, rmk+kernel@...linux.org.uk,
        geert+renesas@...der.be, linus.walleij@...aro.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        sebastian.reichel@...labora.com, Mike Rapoport <rppt@...nel.org>,
        aneesh.kumar@...ux.ibm.com,
        the arch/x86 maintainers <x86@...nel.org>,
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        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to MM CPUs

> 
> On Jun 20, 2023, at 7:46 AM, Yair Podemsky <ypodemsk@...hat.com> wrote:
> 
> @@ -1525,7 +1525,7 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v
> 				addr + HPAGE_PMD_SIZE);
> 	mmu_notifier_invalidate_range_start(&range);
> 	pmd = pmdp_collapse_flush(vma, addr, pmdp);
> -	tlb_remove_table_sync_one();
> +	tlb_remove_table_sync_one(mm);

Can’t pmdp_collapse_flush() have one additional argument “freed_tables”
that it would propagate, for instance on x86 to flush_tlb_mm_range() ?
Then you would not need tlb_remove_table_sync_one() to issue an additional
IPI, no?

It just seems that you might still have 2 IPIs in many cases instead of
one, and unless I am missing something, I don’t see why.